Mica insulator



Dec. 18, 1934. w H. BANKS, JR.. ET AL 1,934,911

' MICA INSULATOR Filed Aug. 19, 1931 IN'v N+URSZ macaw Patented Dec. 18, 1934 UNITED STATES PATENT OFFICE Graham, Saugus, Mass.,

assignors to The Macallen Company, Boston, Mass., a corporation of Massachusetts Application August 19,

1931, Serial No. 558,031

This invention relates to articles made of mica,

and particularly to treating such articles by coating them with a thin covering of chemically treated cellulose, such as cellulose acetate, viscose'or regenerated cellulose.

As is well known, mica'is in wide use for making parts of electrical equipment which must be non-conductors of electricity, such as commutator rings and segments, tape for armature windings, and insulation tubes. As the natural mica does not possess the necessary area, thickness, cohesion and pliability for making such' insulating parts, it is customary to prepare composite sheets of mica cemented together with a binder from which the desired parts can be made. This *iS usually done, either manually or by machine, by showering mica in small flakes upon a table or other suitable surface, cementing together the particles by adding a binder in liquid form, such as shellac, copal or glyptol, rolling the mixture, and repeating the operation until a composite sheet of the. desired thickness is built up. The insulating parts are then cut out, die shaped, or otherwise worked into the desired articles from the composite sheets of mica.

The articles thus prepared make suitable in? sulating pieces for use in electrical equipment, so far as their insulating qualities are concerned, but they are not entirely satisfactory as they are subject to defects inherent in their nature, such as the flaking off or feathering of the mica, and sticking of the pieces in assembling machines. The tendency of the mica to flake of! greatly shortens the life of the insulating pieces, as excessive handling causes them to'disintegrate. The mica tape for winding armatures likewise is defective because as the mica particles are small and do 'not overlap to any great extent, thetape will easily break or tear.

The object of our invention is to eliminate and overcome these defects, and this has been achieved by applying -a*-'coating of cellulose acetate, viscose, or the like to the mica com msition, which adheres to the binder in the composition.

Other objects of this invention will appear in the following description andappended claims,

, reference being had to theaccompanying drawing forming )a part of this specification.

Fig. 1 is across sectional view of a commutator ringcoated on-its top side with the covering film.

, Fig. 2 is a perspective iew of a strip of mica tape coated on both sides.

Fig. 3 is a lateral cross section thereof.

. Before explaining in detail the present invention, it is to be understood that the invention is not limited in its application to the details of construction and arrangement of parts illustrated in the accompanying drawing, since the invention is capable of other embodiments and of being practised or carried out in various ways. Also it is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. and it is not intended to limit the invention beyond the terms of the several claims hereto appended as considered in view of the prior art and the requirements thereof.

. Referring to Fig. 1, 10 represents the composite mixture of mica andshellac and 11 the protective coveringor film of cellulose acetate, viscose or the like, commonly known as Cellophane, and which materials are obtained by chemically treating cellulose. The coating of cellulose acetate or regenerated cellulose is preferably applied to the top side of the composite sheet of mica before the pieces are cut or stamped out therefrom, but it may be applied to both sides and to the piece after it has been stamped out, if desired. The protective coating of cellulose acetate or regenerated cellulose easily adheres to the mica because of the adhesive binder in the mica sheet, but an extra coating of the binder is usually used to make the covering adhere better to the mica sheet. With the protective coating of cellulose acetate on the composite sheet, and on the-commutator rings and segments, and other insulating pieces stamped therefrom, the efiective life of the pieces is greatly lengthened, as the coating protects them from flaking oif from handling. The protective covering is ordinarily transparent, which is an advantage in itself.

In Figs. 2 and 3 are shown a perspective an cross sectional view of a strip of mica tape similarly coated. The mica tape is made much thinner than the usual composite sheet and the addition of the protective coating on both sides greatly increases the strength of the tape.

Numerous other insulating parts may be made, the pieces shown in the drawing being used for purposes of illustration only. When mica commutator segments are given the protective covering of cellulose acetate, the segments are particularly adapted for use in commutator assembling machines which assemble one segment of mica and one of copper alternately. The protective covering gives the surface of the mica segment a very smooth finish" so that individual mica segments do not stick together and impair the operation of the machine. It is very important in an assembling machine that the mica segments do not stick together, as the segments must be assembled in alternate relation of mica and copper.

The addition of the protective coating to the composite sheet likewise greatly facilitates the making of the commutator segments, as it has hitherto been found impractical to feed them automatically to a press because of the tendency of the sheets to interlock.

As the protective coating of cellulose acetate or the like is non-conductive of electricity, the insulating properties of the mica piece are not affected by the coating. Of the many substances tried, we have found cellulose acetate and regenerated cellulose most satisfactory for achieving all the desired results. Where chemically treated cellulose is mentioned in the claims, it is understood to include cellulose acetate, regenerated cellulose made from viscose and other cellulosic compounds or esters, as well as other cellulosic materials having equivalent characteristics.

We claim:

1. As an article of manufacture, a composite sheet of mica and a binder, and a supporting and covering film therefor, said film comprising a preformed, substantially continuous sheet of chemically treated, substantially transparent cellulose having substantially the characteristics of cellulose acetate and/or regenerated cellulose.

2. As an article of manufacture, a composite sheet of mica and a binder and a supporting and covering film on both faces thereof, each of said films comprising a preformed, substantially continuous sheet of chemically treated, substantially transparent cellulose having substantially the characteristics of cellulose acetate and/or regenerated cellulose. I

WILLIAM H. BANKS, JR. WALTER A. GRAHAM. 

